[1.0] Keep private input data out of public WorkflowError artifacts

Task failures currently copy str(exc) into a WorkflowError artifact without an explicit restrictive visibility policy. An exception can therefore turn a private input into a public error payload.

Scope

• Publish a content-minimal public error code and keep detailed diagnostics behind the appropriate reader permissions.
• For multiple inputs, derived diagnostics require their common permitted readership; unknown or ambiguous attribution must not broaden access.
• Cover context construction, execution and output/consumption persistence failures without losing the failure's run association.

Acceptance criteria

• A deterministic engine raises a synthetic private input value; foreign and anonymous readers cannot retrieve it from error artifacts or exposed diagnostics.
• The authorized operator can identify the failed run and inspect the permitted details.
• Mixed-visibility inputs do not widen diagnostic readership, and failure handling cannot itself publish raw exception content publicly.

Boundaries

Automatic retries, durable approvals and a new general policy framework are deferred.

References

• Parent topic: #280.
• src/flock/core/orchestrator.py
• src/flock/core/artifacts.py
• src/flock/core/visibility.py

Start by reading src/flock/core/orchestrator.py, src/flock/core/artifacts.py, and src/flock/core/visibility.py, then trace context construction, execution, and output or consumption persistence failures. Done means public artifacts expose only a deterministic minimal error code, permitted operators retain run association and authorized details, and foreign or anonymous readers cannot retrieve private exception content, including for mixed-visibility inputs.